standard generation and calibration system for volatile

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Standard generationand calibration

system for volatileorganic compounds

P. Veres et al.

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Atmos. Meas. Tech. Discuss., 3, 333–357, 2010www.atmos-meas-tech-discuss.net/3/333/2010/© Author(s) 2010. This work is distributed underthe Creative Commons Attribution 3.0 License.

AtmosphericMeasurement

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This discussion paper is/has been under review for the journal Atmospheric Measure-ment Techniques (AMT). Please refer to the corresponding final paper in AMTif available.

Development and validation of a portablegas phase standard generation andcalibration system for volatile organiccompoundsP. Veres1,2, J. B. Gilman2,3, J. M. Roberts2, W. C. Kuster2, C. Warneke2,3,I. R. Burling4, and J. de Gouw2,3

1Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA2Chemical Sciences Division, Earth System Research Laboratory, National Oceanic andAtmospheric Administration, Boulder, CO 80305, USA3Cooperative Institute for Research in Environmental Sciences, University of Colorado,Boulder, CO 80309, USA4University of Montana, Department of Chemistry, Missoula, USA

Received: 8 January 2010 – Accepted: 11 January 2010 – Published: 29 January 2010

Correspondence to: P. Veres ([email protected])

Published by Copernicus Publications on behalf of the European Geosciences Union.

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Abstract

We report on the development of an accurate, portable, dynamic calibration systemfor volatile organic compounds (VOCs). The Mobile Organic Carbon Calibration Sys-tem (MOCCS) combines the production of gas-phase VOC standards using perme-ation or diffusion sources with quantitative total organic carbon (TOC) conversion on a5

palladium surface to CO2 in the presence of oxygen, and the subsequent CO2 mea-surement. MOCCS was validated using three different comparisons: (1) TOC of highaccuracy methane standards compared well to expected concentrations (3% relativeerror), (2) a gas-phase benzene standard was generated using a permeation sourceand measured by TOC and gas chromatography mass spectrometry (GC-MS) with10

excellent agreement (<4% relative difference), and (3) total carbon measurement of4 known gas phase mixtures were performed and compared to a calculated carboncontent to agreement within the stated uncertainties of the standards. Measurementsfrom laboratory biomass burning experiments of formic acid by negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) and formaldehyde by15

proton transfer reaction-mass spectrometry (PTR-MS), both calibrated using MOCCS,were compared to open path Fourier transform infrared spectroscopy (OP-FTIR) tovalidate the MOCCS calibration and were found to compare well (R2 of 0.91 and 0.99respectively).

1 Introduction20

Volatile organic compounds (VOCs) are emitted to the atmosphere from a variety ofsources, both natural and man-made. In the atmosphere, the photo-oxidation of VOCsleads to formation of ozone and organic aerosol, which are both significant pollutantsand alter radiative forcing in the Earth’s climate system. Accurate measurements ofambient VOCs in the atmosphere or laboratory using analytical techniques such as25

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gas-chromatography (Rappengluck et al., 2006) or PTR-MS (de Gouw and Warneke,2007) rely on the accuracy of the calibration methods used. For many VOCs, but es-pecially for “sticky” compounds that have a high affinity for metal surfaces, accuratecalibration systems are not readily available. VOC standards for instrument calibrationare generally produced either statically or dynamically. Static methods rely on mix-5

tures of gases in closed containers of known volume, while dynamic processes involvemixing a continuous flow of analyte into a dilution or carrier stream. A comprehensivereview of standard generation processes for VOC can be found elsewhere (Barratt,1981; Namiesnik, 1984; Naganowska-Nowak et al., 2005)

Modern static techniques most commonly utilize mixtures of gases in treated high-10

pressure cylinders (Apel et al., 1994, 1998; Rappengluck et al., 2006). Static tech-niques are the preferred calibration method for many field and laboratory investiga-tions due to the portability and robust nature of high-pressure gas cylinders. Standardmixtures are stable for many VOCs; however, wall losses and degradation becomesignificant for highly polar and reactive compounds.15

Dynamic standard generation can avoid the problem of wall losses and degradationby continuously flowing analyte into a carrier stream. This method is more suitable forpolar and reactive species which would otherwise be lost to surfaces at low mixing ra-tios (Barratt, 1981). The two most commonly used techniques are standard generationthrough the use of diffusion cells (Thompson and Perry, 2009; Altshuller and Cohen,20

1960; Possanzini et al., 2000; Namiesnik et al., 1981; Williams et al., 2000) and perme-ation sources (Okeeffe and Ortman, 1966). Diffusion sources are often used in placeof permeation sources when the latter is unavailable or behavior of a substance in apermeation source is nonideal (i.e. degredation, low permeation rate, etc.) (Barratt,1981). A combination of static and dynamic standard generation is also often used25

where a high mixing ratio (ppmv) static mixture is diluted dynamically. This allows forconvenient portability but losses are not as significant as with very low mixing ratiostatic sources.

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Total carbon measurement by conversion to CO2 and nondispersive infrared (NDIR)sensor analysis has long been used for analysis of a range of environmental samplesranging from natural waters to bulk collected aerosol particles. Total organic carbon(TOC) measurements of gas phase nonmethane hydrocarbons using oxidative cata-lysts, followed by reduction to methane, have been used in previous work (Maris et5

al., 2003; Roberts et al., 1998). Ambient VOC levels are frequently below the detectionlimits of these gas phase TOC measurement techniques, which need to account for thelarge concentrations of ambient CO2, methane, and carbon monoxide and make themunsuitable for ambient analyses. The TOC technique is however particularly well suitedfor the analysis of calibration standards where the mass loading can be user-varied to10

fall within a measureable range and the effects of carrier gas CO2 can be eliminated.While the technique used here is similar to TOC methods utilizing the catalytic con-version of organic carbon, we use a direct measurement of the CO2 produced in thisprocess as a calibration measurement.

In this work we developed a mobile organic carbon calibration system (MOCCS) for15

the generation and absolute measurements of calibrated VOC mixtures in air that isrelatively inexpensive and easy to set up. The MOCCS combines the production ofstandards using permeation or diffusion sources, quantitative catalytic conversion ofcarbon containing species to CO2, and subsequent CO2 measurement. Validation ofthis technique was performed in a three-part analysis: (1) two high accuracy methane20

standards were analyzed and compared to their known concentrations, (2) comparisonof measurements by gas chromatography mass spectrometry (GC-MS) and MOCCS ofa benzene standard generated using a permeation source with the system describedhere, and (3) several complex gravimetrically prepared VOC standards were analyzedfor total carbon content.25

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2 Experimental details

2.1 Mobile Oxidative Carbon Calibration System (MOCCS)

MOCCS, shown in Fig. 1, consists of 16 flow and temperature controlled permeationtube housings, a thermostated palladium catalyst at 350 ◦C to readily convert VOCs toCO2, and a CO2 detector (Beckman Industrial Model 870 NDIR or LI-COR LI-6252).5

The entire assembly was mounted in a portable rack that includes a high-pressure zeroair cylinder and an uninterruptable power supply. These features allow MOCCS to betemporarily removed from power for transport while providing continuous temperaturecontrol and flow over the permeation sources.

The housing was designed to hold 16 permeation sources at a controlled flow. Four10

aluminum blocks were drilled for 1/2′′ o.d. PFA Teflon sleeves to house four perme-ation sources each. A temperature probe was inserted in the center of each blockand the temperature regulated using individual controllers. Each permeation housingcan be operated at a separate temperature with each block housing up to four perme-ation sources. Sections of 1/16′′ o.d. stainless steel capillary tubing ∼15 cm long were15

crimped until a flow of ∼10 sccm was provided to each of the 16 channels at 30 psi.Due to slight variations in the inlet pressure, the flow is measured for each channel atthe time of calibration.

A typical permeation source calibration is based on the mass loss of the perme-ation tube and usually requires running for several weeks under stable conditions.20

MOCCS measures real time emission rates of permeation sources after equilibratingwithin hours. As a result, any constant VOC source can be used thereby eliminatingthe necessity of obtaining certified permeation sources that can be quite costly. In thiswork, permeation sources were made in-house using pure compounds placed into 1/4′′

or 1/8′′ Teflon permeation tubes purchased from VICI Metronics and sealed with Teflon25

plugs with crimped stainless steel bands. Adjusting the temperature of the permeationhousing can easily vary the output of the sources.

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The catalyst is comprised of a 3′′ length 1/4′′ o.d. stainless steel tube packed with10% Pd on Kaowool (Johnson-Matthey, Ward Hill, MA.) and capped with a smallamount of glass wool, to prevent the palladium from exiting the catalyst. A thermo-couple is attached to the midpoint of the stainless steel tubing and then wrapped ina single layer of insulated Nichrome wire. The finished catalyst is well insulated and5

placed into an aluminum box with 1/4′′ Swagelok bulkhead unions. A 24 Volt tempera-ture controller is used to supply power to the Nichrome wire and set the temperature to350◦C. This catalyst design is based on that used for PTR-MS background measure-ments (de Gouw and Warneke, 2007).

The flow system was designed for the sample stream to be analyzed for both back-10

ground CO2 in the carrier gas and total organic carbon. A schematic of the flowsystem is shown in Fig. 1. The system was designed to continuously cycle betweenbackground CO2 measurement and standard calibration on a user-defined timescale.Background CO2 measurements are made by flowing the carrier gas through the per-meation directly source into the CO2 analyzer bypassing the catalyst. Measuring the15

background in this manner accounts for any CO2 present in the carrier gas as well asany potential interference from the IR absorption of VOCs. This background is sub-tracted from the total organic carbon measurement. The presence of VOCs in thecarrier gas must also be corrected for by subtracting a TOC measurement of the car-rier gas (∼400 ppbv) prior to addition of a permeation source. The catalyst is kept20

under constant flow by flushing with 10 sccm zero air when the background is beingmeasured.

A Beckman Industrial Model 870 NDIR was used to measure the CO2 concentrationsfor most results presented here. Four CO2 standards in ultrapure air (Scott-Marin Inc.)ranging from 2.065±0.207 ppmv up to 50.1±0.5 ppmv were used to calibrate the NDIR.25

The results of the calibration are shown in Fig. 2. The precision in NDIR CO2 mea-surements through the calibrated concentration range was ±1% of the measurement+30 ppbv. The accuracy of CO2 measurements through the 2 ppmv to 50 ppmv con-centration range was ±4% of the measurement +400 ppbv. Significant improvements

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in both precision and accuracy were made by substitution of a LI-COR LI-6252 CO2analyzer and will be presented later in this discussion.

This TOC technique has been previously used to calibrate standards from diffusionsources for organic acids in the development of negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) (Veres et al., 2008). The diffusion flow sys-5

tem used in that particular study was identical to that described in Williams et al. (2000).

2.2 GC-MS

A custom built gas chromatograph with a quadrupole mass spectrometer detector (GC-MS) was used to independently verify the concentrations of two benzene standardsgenerated and calibrated by the MOCCS. A detailed description of the GC-MS is de-10

scribed by Goldan et al. (2004). The output of the benzene permeation tube was firstdiluted in humidified nitrogen by factors of 9.78 (±0.11)×10−4 and 8.62 (±0.11)×10−4.This was done in order to avoid overloading the GC-MS, which normally operates in thepptv to ppbv range. The GC-MS collected each sample directly from the diluted samplestream before subsequent analysis. A minimum of 10 replicate samples were analyzed15

for each benzene concentration with an overall measurement precision of 3% or better.Benzene measured by the GC-MS was independently calibrated using more than 20single- and multi-component VOC mixes with an overall measurement uncertainty of±20%.

2.3 NI-PT-CIMS20

Negative-ion proton-transfer chemical-ionization Mass Spectrometry (NI-PT-CIMS)provides gas-phase acid measurements with one-second time resolution. A detaileddescription of NI-PT-CIMS can be found elsewhere (Veres et al., 2008). Briefly, NI-PT-CIMS consists of (1) a 210Po source to produce acetate ions (CH3C(O)O−) from aceticanhydride, (2) a flow tube reactor, in which CH3C(O)O− undergoes proton transfer25

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reactions with inorganic and organic acids, (3) a collisional dissociation chamber (CDC)to decluster ions, and (4) a quadrupole mass spectrometer for the detection of bothreagent and product ions.

2.4 OP-FTIR

The open path Fourier transform infrared (OP-FTIR) instrument included a Bruker5

Matrix-M IR Cube spectrometer and a thermally stable open White cell. The Whitecell path length was set to 58 m. The spectral resolution was set to 0.67 cm−1 andthe spectrometer acquired spectra every 1.5 s (four co-added spectra). A pressuretransducer and two temperature sensors were located adjacent to the optical path andwere logged on the instrument computer and used for spectral analysis. Mixing ratios10

were obtained by multi-component fits to sections of the IR transmission spectra witha synthetic calibration non-linear least-squares method (Griffith, 1996; Yokelson et al.,2007).

2.5 PTR-MS

Proton transfer reaction-mass spectrometry (PTR-MS) utilizes proton-transfer reac-15

tions of H3O+ to detect various atmospheric trace gases, usually as the MH+ion.PTR-MS allows for the detection of numerous volatile organic compounds with highsensitivity (10–100 pptv) and response time (1–10 s). This technique has been usedextensively in aircraft, ground-based and laboratory studies. A more complete discus-sion of the PTR-MS system used in this study can be found elsewhere (de Gouw and20

Warneke, 2007).

2.6 Gas standards

A detailed list of the standards used in this work is shown in Table 1. VOC mix 1–4 arehigh-pressure gas cylinder standards prepared at NOAA ESRL/CSD laboratory usinggravimetric techniques. The error in each component in these laboratory-generated25

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standards is estimated to be no greater than 20%. Two high accuracy methane stan-dards that were prepared gravimetrically with an uncertainty of ±0.2% (Dlugokencky etal., 2005) were borrowed from NOAA’s Global Monitoring Division (Boulder, CO) andanalyzed for TOC. The stated standard concentrations of CH4 (a) and CH4 (b) were5.75±0.11 ppmv and 10.79±0.22 ppmv respectively. A laboratory-made benzene per-5

meation source was equilibrated and calibrated by MOCCS for analysis.

3 Validation of the MOCCS

3.1 Data analysis

The results of the measurement of a benzene standard generated using the MOCCSas an example is shown in Fig. 3. Interpolated background measurements are sub-10

tracted from the total organic carbon measurements to obtain a concentration for thesource compound. Dividing the result by the number of carbons in the parent molecule(6) gives the original standard concentration on a molar basis, assuming a conversionratio of 1:1 for the oxidation of carbon to CO2. The signal after the initial response timeis then averaged to obtain an average standard concentration for each cycle. The inset15

in Fig. 3 shows the stability of the MOCCS that had a measured precision of betterthan 1% over 5 days. All of the TOC measured concentrations reported here are theaverages of a minimum of 10 cycles. The duration of each cycle can be readily ad-justed to allow for different time responses of the system to various compounds, whichis dependent on their unique chemical properties (e.g. volatility, polarity, etc.).20

3.2 Methane conversion to CO2

The results of the two comparisons for the methane standards are shown in Fig. 4aas CH4 (a) and CH4 (b). The TOC determined methane concentration was deter-mined as 5.9±0.6 ppmv for CH4 (a) and 10.8±0.8 ppmv for CH4 (b). The relative

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errors in concentration between the TOC system and provided standard concentra-tion were less than 3% in both cases and the two determinations agreed within thestated uncertainties.

3.3 Benzene conversion to CO2

A benzene permeation source in the MOCCS was measured using both GC-MS and5

TOC. The results of the comparison are shown in Fig. 4b. Two permeation temper-ature settings were used to generate different concentrations, 7.36±0.14 ppmv and3.79±0.10 ppmv as measured by TOC. The source output was diluted for analysiswith GC-MS: dilution factors of 9.78 (±0.11)×10−4 and 8.62 (±0.11)×10−4 were used.GC-MS-measured benzene concentrations, after taking dilution into account, were10

7.11±1.42 ppmv and 3.65±0.73 ppmv respectively. The relative difference betweenTOC and GC-MS measurements is less than 4% for both standards measured.

3.4 TOC of VOC standard mixtures

Results of the total carbon analysis are shown in Fig. 5 for each of the VOC mixturesavailable. The CO2 concentration reported is the total carbon concentration of the stan-15

dards (striped bars), also detailed in Table 1. Error in the NOAA generated standards isestimated to be about 10% based on the propagation of uncertainties in the gravimet-ric preparation method. TOC measured concentrations are shown in solid grey. Error(1σ) in the measurements shown in Fig. 4 represent the RMS of the standard deviationfrom the average of multiple TOC cycles (5%), the error associated with the calibration20

of the Beckman NDIR ±4% of the measurement +400 ppbv, and error in the dilutionflows (2%). The agreement for all measurements is within the estimated error of thelaboratory standards. The absolute error in the calculated total VOC concentration inthe four standards analyzed as measured by MOCCS is less than 15%.

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3.5 MOCCS field calibration

MOCCS was recently used during the Smoke Understanding through Regional FireSimulations (SMURFS) 2009 study preformed at the combustion facility at the US De-partment of Agriculture (USDA) Forest Service, Fire Sciences Laboratory (FSL) in Mis-soula, MT. A more in-depth discussion of the SMURFS 2009 study can be found in a fu-5

ture publication (Veres et al., 2010). Briefly, emissions of controlled laboratory biomassfires were sampled directly from a stack in which the fire emissions were completely en-trained. A negative-ion proton-transfer chemical ionization mass spectrometer (NI-PT-CIMS) was calibrated for formic acid using a permeation standard that was generatedand calibrated with MOCCS. A proton-transfer-reaction mass spectrometer (PTR-MS)10

was calibrated for formaldehyde measurements using MOCCS. Simultaneous mea-surements of both formic acid and formaldehyde were made using open-path Fouriertransform infrared spectrometer (OP-FTIR) and were used to validate the MOCCS cal-ibration of NI-PT-CIMS and PTR-MS. Figure 6 shows time profile for both formic acid,measured by NI-PT-CIMS and OP-FTIR, and formaldehyde, measured by PTR-MS and15

OP-FTIR.The results of a comparison of NI-PT-CIMS and OP-FTIR formic acid emission mea-

surements from a single laboratory biomass fire are shown in Fig. 6a. A scatter plotof the data shown gives a NI-PT-CIMS/OP-FTIR formic acid ratio of 0.91±0.02 with acorrelation (R2) of 0.91. This agreement is well within the stated uncertainty of both20

instruments. Figure 6b shows the results of a comparison of PTR-MS and OP-FTIRformaldehyde emission measurements from the same laboratory controlled biomassfire shown in Fig. 6a. The PTR-MS/OP-FTIR formaldehyde ratio is 1.06±0.02 with acorrelation (R2) of 0.99 from the corresponding scatter plot. The detection of formalde-hyde by PTR-MS is humidity dependent (Hansel et al., 1997); however, the humidity25

dependence is ignored here, the relative humidity during calibration and fire measure-ments were similar for this particular experiment. A study of the sensitivity of PTR-MSfor formaldehyde as a function of humidity is beyond the scope of the present work.

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The agreement between PTR-MS and OP-FTIR in this comparison is well within theestimated uncertainty of both instruments. The agreement between these two setsof independently calibrated measurements validates the use of the MOCCS for VOCstandard production and calibration.

3.6 Comparison of CO2 analyzers5

A recent improvement to the MOCCS was the replacement of the Beckman model 870NDIR CO2 analyzer with a LI-COR LI-6252 CO2 analyzer. Table 2 summarizes theresults of a comparison of the two CO2 analyzers. Significant improvements in the in-strument response times and detection limits are observed with the LI-COR analyzer.Figure 7 shows the results of a comparison in formic acid response times and detec-10

tion limits as defined by one complete MOCCS cycle. The formic acid response time,defined as the time required for a calibration signal to decay to 10% of the initial valuewhen the source is removed, was approximately 90 s for the LI-COR compared to over300 s for the Beckman analyzer. The detection limit for the compounds investigated inthis study was determined from a signal-to-noise ratio of 2 as twice the standard devi-15

ation in the background. The LI-COR detection limit of 5 ppbv was significantly lowerthan that of the Beckman analyzer, 150 ppbv. The precision in LI-COR CO2 measure-ments through the calibrated concentration range is ±1% of the measurement +1 ppbv.The accuracy of LI-COR CO2 measurements through the 2 ppmv to 50 ppmv concen-tration range is ±1% of the measurement +80 ppbv. In addition to improvements in20

precision, accuracy and detection limit, the overall error from the average of multipleTOC cycles is also reduced from 5% of the measurement to 1% when using the LI-COR analyzer. These improvements from use of the LI-COR analyzer compared to theBeckman NDIR analyzer are significant and show that the accuracy of this techniqueis highly dependent on the type of CO2 analyzer used.25

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4 Conclusions

A portable system for the dynamic production and calibration of gas phase VOCs hasbeen developed. We use a combination of catalytic reduction of VOCs to CO2 and thesubsequent measurement of the CO2 produced by an NDIR analyzer to standardizecalibration sources. We have validated the TOC measurement technique through total5

carbon analysis of two high accuracy methane standards showing excellent agreementwith 3% absolute error. MOCCS was validated further by the production and subse-quent calibration of a benzene standard by GC-MS and TOC with excellent agreement(<4% relative difference). Four laboratory prepared mixed VOC gas-phase standardswere analyzed for TOC and shown to be accurate to within 15%. The MOCCS sys-10

tem is relatively inexpensive to develop in laboratory. It allows for dynamic generationand calibration of pure compounds, such as acids, which are not well suited for othercommonly used standard generation processes. Additionally, MOCCS is particularlywell suited for field deployment, as it is completely mobile. This technique representsa novel advancement in gas phase standard production and calibration for both VOCs15

and potentially SVOCs.

Acknowledgements. The authors would like to thank Edward Dlugokencky for supplying theGMD methane standards used. We also want to thank Robert J. Yokelson for his role in theSMURFS 2009 study. This work was supported by the NOAA’s Health of the Atmosphere Pro-gram and NOAA’s Climate Goal, NSF Grant # ATM 1542457, the CIRES Innovative Research20

Program, and DOD SERDP Grant Nos. SI-1648 and SI-1649.

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Williams, J., Roberts, J. M., Bertman, S. B., Stroud, C. A., Fehsenfeld, F. C., Baumann, K., Buhr,M. P., Knapp, K., Murphy, P. C., Nowick, M., and Williams, E. J.: A method for the airbornemeasurement of pan, ppn, and mpan, J. Geophys. Res.-Atmos., 105, 28943–28960, 2000.

Yokelson, R. J., Karl, T., Artaxo, P., Blake, D. R., Christian, T. J., Griffith, D. W. T., Guenther, A.,and Hao, W. M.: The Tropical Forest and Fire Emissions Experiment: overview and airborne25

fire emission factor measurements, Atmos. Chem. Phys., 7, 5175–5196, 2007,http://www.atmos-chem-phys.net/7/5175/2007/.

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Table 1. Standard Gas Mixtures.

Standard Name Contents Concentration [Carbon ](ppmv, ±20%a ) (ppmv)

VOC Mix 1 2-methylfuran 10.01 413.21Methyl vinyl ketone 7.53Benzene 10.53Furfural (2-furanaldehyde) 10.68a-methyl styrene 12.44p-cymene 10.44

VOC Mix 2 Acetaldehyde 2.01 89.76Methanol 1.96Isoprene 1.97Acetone 1.89Acetonitile 2.01Methacrolein 2.092-butanone 1.92Benzene 1.97b-pinene 1.911,3,5-Trimethylbenzene 1.90

VOC Mix 3 MBO 10.73 463.812,3-Butanedione 5.822-Butenal 8.56Toluene 10.62Decene 8.69Benzofuran 11.01Indene 11.46

aThe error in the concentration of the species listed is assumed to be 20% unless otherwisestated.

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Table 1. Continued.

Standard Name Contents Concentration [Carbon ](ppmv, ±20%a ) (ppmv)

VOC Mix 4 Acetylene 13.41 274.95Propene 11.301,3-Butadiene 10.54MTBE 9.50Benzene 9.41Benzaldehyde 9.73

CH4 (a) Methane 5.752 ± 0.11 5.75

CH4 (b) Methane 10.79 ± 0.22 10.79

aThe error in the concentration of the species listed is assumed to be 20% unless otherwisestated.

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Table 2. Performance of MOCCS with the Beckman Industrial Model 870 NDIR and the LI-CORLI-6252.

Beckman 870 LI-6252

Response time∗ 300 s 90 s

TOC Detection Limit 150 ppbv 5 ppbv

TOC Accuracy ±4% of the measurement ±1% of the measurement+400 ppbv +80 ppbv

TOC Precision ±1% of the measurement ±1% of the measurement+30 ppbv +1 ppbv

Error on replicate TOC cycles 5% 1%

∗Response times are listed for a formic acid permeation source.

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13

671 Figure 1. 672

673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706

Fig. 1. A schematic diagram of the TOC calibration system. Gas standards are selectivelypassed over a palladium catalyst (CC). The CO2 generated via oxidation of a standard is thenmeasured using NDIR. During a background measurement, valves 1 and 4 open to allow flowto bypass the catalyst. At this time, valves 2 and 3 are closed shut and valves 5 and 6 areopened to flush the catalyst with zero air. To make a total carbon measurement, valves 2 and3 opened, 5 and 6 closed, and valves 1 and 4 are switched to allow the sample to flow throughthe catalyst with the outflow to the CO2 detector.

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707 Figure 2. 708 709 710 711 712

713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734

Fig. 2. Calibration of the Beckman 870 NDIR CO2 analyzer. Four CO2 gas standards wereused: 2.065 (±0.207) ppmv, 5.12 (±0.10) ppmv, 20.07 (±0.40) ppmv, and 50.1 (±0.5) ppmv.

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735 Figure 3. 736 737 738 739

740 741 742 743 744 745

15

735 Figure 3. 736 737 738 739

740 741 742 743 744 745 Fig. 3. CO2 measurements showing two complete cycles of background and TOC measure-

ment of a benzene standard generated by MOCCS, (a) The TOC measurements are shown inblack with open circles and the background measurements are shown in grey. Subtracting theinterpolated background and dividing the result by the number of carbon atoms in a benzenemolecule (6) gives the benzene concentration from the permeation source. After the signalstabilizes, an average of the measured signal minus background is taken (shown as the databetween the dashed line). Shown in (b) are replicate measurements of the benzene concen-tration made over the course of five days. The average concentration over this time period (38cycles) was determined to be 13.8±0.1 ppmv of carbon (ppmvC).

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746 Figure 4. 747 748

749 750 751 752 753 754 755 756 757

16

746 Figure 4. 747 748

749 750 751 752 753 754 755 756 757

Fig. 4. Results of MOCCS determined methane concentration compared to GMD determinedmethane concentration of two standards, (a). The relative difference between the two measure-ments is 3%.The results of GC-MS/TOC measurements of two benzene standards generatedusing MOCCS, b). The relative difference between the two measurements is less than 4%.

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758 Figure 5. 759 760 761 762

763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786

Fig. 5. The results of a total carbon analysis of 4 gas phase VOC standards. The contents ofeach mixture are detailed in Table 1. An uncertainty of 20% was assumed for each componentin the laboratory-generated VOC standards.

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787 Figure 6. 788 789 790

791 792 793 794 795 796 797 798 799

Fig. 6. A comparison of NI-PT-CIMS formic acid and PTR-MS formaldehyde measurements tosimultaneous measurement of OP-FTIR formic acid and formaldehyde. NI-PT-CIMS was cali-brated using a formic acid permeation source that was standardized using MOCCS. PTR-MSwas calibrated for formaldehyde measurement with a permeation source that was standardizedusing MOCCS.

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800 Figure 7. 801 802 803 804 805 806

807

Fig. 7. Formic acid measurement comparison using MOCCS with the Beckman model 870NDIR CO2 analyzer and the LI-COR LI-6252 CO2 analyzer. The TOC system was set to a1-hour cycle between CO2 catalysis measurements and background measurement. Significantimprovements in both CO2 detection limit and instrument response times are achieved with theLI-COR LI-6252 analyzer. It is important to note that the two CO2 measurements are not of thesame formic acid standard, which explains the difference in left and right axes.

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